Fix ZipOS deadlock/segfault (#1011)

This change adds a new stress test for ZipOS which helped
us improve the locking semantics in open() and close().

libc/runtime/zipos-read.c

@@ -58,6 +58,7 @@ static ssize_t __zipos_read_impl(struct ZiposHandle *h, const struct iovec *iov,
     if (b) memcpy(iov[i].iov_base, h->mem + y, b);
   }
   if (opt_offset == -1) {
+    unassert(y != SIZE_MAX);
     atomic_store_explicit(&h->pos, y, memory_order_release);
   }
   return y - x;